The invention relates generally to the field of balanced mixer circuits and more particularly to balanced dual output diode mixer circuits adaptable for mixing two RF (radio frequency) input signals and producing audio or RF output signals.
Balanced mixer circuits, also commonly referred to as ring modulator circuits, generally comprise two transformers each having a center tapped secondary winding, and a diode or other switching type network coupled between the end terminals of the center tapped windings. Typically, a first RF singal is used to excite the primary winding of one transformer and a second RF excitation signal is coupled to the center tap of one of the secondary windings of the transformers. The diode switching network produces a non-linear periodic passing and blocking of signals between the transformers. This results in output signals, having frequencies equal to the sum and difference of the two RF excitation signal frequencies, being obtainable from the primary winding of the second transformer. Such balanced mixers are known and used to produce RF output signals which are substantially isolated from both of the two RF excitation signals.
These prior art balanced mixers have the substantial disadvantage of not being able to produce both an RF output difference signal and an audio output difference signal since available transformers cannot perform satisfactorily at both of these frequency ranges. Thus in many applications, such as in a single sideband radio, these prior balanced mixers cannot be used, without extensively switching input and output connections, to produce both the detected audio signals from the receiver IF (intermediate frequency) signals while also being used to produce a transmitted RF signal by mixing a modulated IF signal with a local oscillator signal.
Some prior art single sideband radios have modified the basic balanced diode mixer and produced a marginally effective mixer which is capable of producing both RF and audio output signals without switching the input and output connections. These dual output balanced mixers use RF bypass (audio blocking) capacitors in series with the signals coupled between the two transformers, and then use RF chokes to tap off the audio frequencies blocked by these capacitors. These prior art circuits connect the RF chokes at points of high RF impedance. The RF chokes therefore degrade the performance of the mixer and high quality RF chokes are required to minimize loading effects. In addition, the capacitors block the rectified DC created by the switching network and thus inherently disturb the operation of the switching network. The net effect is a degradation of the isolation between the two RF excitation signals and the resultant RF and audio output signals as well as an increase in the magnitude of the spurious signals produced by these type of balanced mixers. Also, the operation of the switching network is substantially disturbed.
Thus prior art balanced diode mixers have not been able to provide both RF and audio output signals without reducing the isolation between the RF input and output signals, creating excessive levels of spurious products which cannot be filtered out, or using complex and costly input and output terminal switching networks.